Conventional Carburization
Conventional (high temperature) carburization is a widely used industrial process for enhancing the surface hardness of shaped metal articles (“case hardening”). In a typical commercial process, the workpiece is contacted with a carbon-containing gas at elevated temperature (e.g., 1,000° C. or more) whereby carbon atoms liberated by decomposition of the gas diffuse into the workpiece's surface. Hardening occurs through the reaction of these diffused carbon atoms with one or more metals in the workpiece thereby forming distinct chemical compounds, i.e., carbides, followed by precipitation of these carbides as discrete, extremely hard, crystalline particles in the metal matrix forming the workpiece's surface. See, Stickels, “Gas Carburizing”, pp 312 to 324, Volume 4, ASM Handbook, © 1991, ASM International.
Stainless steel is corrosion-resistant because, as soon as the surface of the steel is exposed to the atmosphere, it immediately forms an impervious layer of chromium oxide. The steel is said to be self-passivating.
When stainless steel is traditionally carburized, the chromium content of the steel is locally depleted through the formation of the carbide precipitates responsible for surface hardening. As a result, there is insufficient chromium in near-surface areas immediately surrounding the chromium carbide precipitates to form the protective chromium oxide on the surface. Since the corrosion resistance of the steel is compromised, stainless steel is rarely case-hardened by conventional (high temperature) carburization.
Low Temperature Carburization
In the mid 1980's, a technique for case hardening stainless steel was developed in which the workpiece is contacted with a carbon-containing gas at low temperature, typically below ˜550° C. At these temperatures, and provided that carburization does not last too long, carbon atoms liberated by decomposition of the gas diffuse into the workpiece surfaces, typically to a depth of 20-50 μm, without formation of carbide precipitates. Nonetheless, an extraordinarily hard case (surface layer) is obtained. Because carbide precipitates are not produced, the corrosion resistance of the steel is unimpaired, even improved. This technique, which is referred to a “low temperature carburization,” is described in a number of publications including U.S. Pat. Nos. 5,556,483, 5,593,510, 5,792,282, 6,165,597, EPO 0787817, Japan 9-14019 (Kokai 9-268364) and Japan 9-71853 (Kokai 9-71853).
Nitriding and Carbonitriding
In addition to carburization, nitriding and carbonitriding can be used to surface harden various metals. Nitriding works in essentially the same way as carburization except that, rather than using a carbon-containing gas which decomposes to yield carbon atoms for surface hardening, nitriding uses a nitrogen containing gas which decomposes to yield nitrogen atoms for surface hardening.
In the same way as carburization, however, if nitriding is accomplished at higher temperatures and without rapid quenching, hardening occurs through the formation and precipitation of discrete compounds of the diffusing atoms, i.e., nitrides. On the other hand, if nitriding is accomplished at lower temperatures without plasma, hardening occurs without formation of these precipitates through the stress placed on the crystal lattice of the metal by the nitrogen atoms which have diffused into this lattice. As in the case of carburization, stainless steels are not normally nitrided by conventional (high temperature) or plasma nitriding, because the inherent corrosion resistance of the steel is lost when the chromium in the steel reacts with the diffusion nitrogen atoms to cause nitrides to form.
In carbonitriding, the workpiece is exposed to both nitrogen and carbon-containing gases, whereby both nitrogen atoms and carbon atoms diffuse into the workpiece for surface hardening. In the same way as carburization and nitriding, carbonitriding can be accomplished at higher temperatures, in which case hardening occurs through the formation of nitride and carbide precipitates, or at lower temperatures in which case hardening occurs through the sharply localized stress fields that are created in the crystal lattice of the metal by the interstitially dissolved nitrogen and carbon atoms that have diffused into this lattice. For convenience, all three of these processes, i.e., carburization, nitriding and carbonitriding, are collectively referred to in this disclosure as “low temperature surface hardening” or “low temperature surface hardening processes.”
Activation
Because the temperatures involved in low temperature surface hardening are so low, carbon and/or nitrogen atoms will not penetrate the chromium oxide protective coating of stainless steel. Therefore, low temperature surface hardening of these metals is normally preceded by an activation (“depassivation”) step in which the workpiece is contacted with a halogen containing gas such as HF, HCl, NF3, F2 or Cl2 at elevated temperature, e.g., 200 to 400° C., to make the steel's protective oxide coating transparent to the passage of carbon and/or nitrogen atoms.
WO 2006/136166 (U.S. Pat. No. 8,784,576) to Somers et al., the disclosure of which is incorporated herein by reference, describes a modified process for low temperature carburization of stainless steel in which acetylene is used as the active ingredient in the carburizing gas, i.e., as the source compound for supplying the carbon atoms for the carburization process. As indicated there, a separate activation step with a halogen containing gas is unnecessary, because the acetylene source compound is reactive enough to depassivate the steel as well. Thus, the carburization technology of this disclosure can be regarded as self-activating.
WO 2011/009463 (U.S. Pat. No. 8,845,823) to Christiansen et al., the disclosure of which is also incorporated herein by reference, describes a similar modified process for nitrocarburizing stainless steel in which an “N/C compound” such as urea, formamide and the like is used as the source compound for supplying the nitrogen and carbon atoms needed for the carbonitriding process. The technology of this disclosure can also be considered to be self-activating, because a separate activation step with a halogen containing gas is also said to be unnecessary.
Surface Preparation and the Beilby Layer
Low temperature surface hardening is often done on workpieces with complex shape. To develop these shapes, some type of metal shaping operation is usually required such as a cutting step (e.g., sawing scraping, machining) and/or a wrought processing step (e.g., forging, drawing, bending, etc.). As a result of these steps, structural defects in the crystal structure as well as contaminants such as lubricants, moisture, oxygen, etc., are often introduced into the near-surface region of the metal. As a result, in most workpieces of complex shape, there is normally created a highly defective surface layer having a plastic deformation-induced extra-fine grain structure and significant levels of contamination. This layer, which can be up to 2.5 μm thick and which is known as the Beilby layer, forms immediately below the protective, coherent chromium oxide layer or other passivating layer of stainless steels and other self-passivating metals.
As indicated above, the traditional method for activating stainless steels for low temperature surface hardening is by contact with a halogen containing gas. These activating techniques are essentially unaffected by this Beilby layer.
However, the same cannot be said for the self-activating technologies described in the above-noted disclosures by Somers et al. and Christiansen et al. in which the workpieces are activated by contact with acetylene or an “N/C compound.” Rather, experience has shown that, if a stainless steel workpiece of complex shape is not surface treated by electropolishing, mechanical polishing, chemical etching or the like to remove its Beilby layer before surface hardening begins, the self-activating surface hardening technologies of these disclosures either do not work at all or, if they do work somewhat, produce results which at best are spotty and inconsistent from surface region to surface region. As a practical matter, therefore, these self-activating surface hardening technologies cannot be used on stainless steel workpieces of complex shape unless these workpieces are pretreated to remove their Beilby layers first.